1. Field of the Invention
The present invention relates to oil refineries, and particularly to a method for optimizing pressure in gas-oil separation plants that uses a genetic algorithm to optimize oil production parameters.
2. Description of the Related Art
Gas-Oil Separation Plants (GOSPs) are very common in oil production facilities. A GOSP typically includes a cascade of vessels through which the pressure of extracted oil is reduced in steps or stages from relatively high well pressure to atmospheric pressure. The selection of the operating pressure of each of these vessels is very important for maximizing hydrocarbon liquid recovery from a given well. The choice of the number of stages and the pressure/temperature of each stage is typically based on laboratory experiments, generally referred to as “separator tests”. These separator tests, however, are time-consuming and costly to perform.
FIG. 2 shows a typical multi-stage separator plant 200. In this plant, the oil is brought from the reservoir with initial reservoir conditions of reservoir pressure Pres and reservoir temperature Tres to the ambient temperature and pressure (Pa, Ta), respectively, in four steps at specified temperatures and pressures; i.e., (P1, T1), (P2, T2), (P3, T3) and (Pa, Ta). At each stage, the liberated gas is collected, and the relevant values are recorded. The initial gas-oil mixture is extracted from the oil reservoir through the oil well 202, where it passes through the first separator or stage 204 with conditions (P1, T1). The liquid is then sent to the second stage 206 (P2, T2) and third stage 208 (P3, T3) sequentially, where the gas is collected again for compression and use as natural gas liquids (NGL plant) 212. Finally, at the last stage 210, the total volume of the collected gas is divided by the remaining liquid in barrels, called “stock tank oil” (STO). The final gas-to-oil ratio (GOR) is referred to as the separator solution GOR, Rs.
During initial testing, a laboratory test, commonly known as the separator test, is performed primarily to determine the oil/gas separation stages to bring oil from the reservoir conditions to the ambient temperature conditions. In oil production, several tests are usually performed using an oil sample at different separator conditions and from differing numbers of separation stages in an effort to ascertain the conditions that can maximize liquid oil production and reduce the amount of escaped gas. The collected gas is considered, in this case, to be a secondary product of lower economic value. On the other hand, the more light components lost in the separator stages, the lower economic value of the remaining oil, as this oil becomes heavier.
The oil specific gravity in the API scale (established by the American Petroleum Institute) is typically used as a measure of the oil quality. A higher value indicates a lighter oil and, thus, a higher market value. Another important performance parameter of the GOSP is known as the “formation volume factor” (FVF), or Bo. The oil formation volume factor is defined as the ratio of the volume of oil at reservoir (in situ) conditions to that at stock tank conditions. This factor is used to determine the well oil flow rate to the production flow rate of the oil (at stock tank conditions).
These three parameters (GOR, API and FVF) are important in determining the operational costs and the estimated revenue of the plant. The operational cost is directly proportional to the well oil production rate. Thus, FVF should be minimized, while the main revenue is proportional to the API of the STO. Thus, API should be maximized. In oil production, gas is considered a byproduct and is either burned on site or collected and sold, but at a lower price than that of oil. As such, GOR should be minimized.
FIGS. 3A, 3B and 3C show oil API, FVF and GOR, respectively, as functions of separator stage pressure, illustrating how these three performance parameters are affected by proper selection of the operating pressure of the separator vessels. It can be clearly seen that adjustment of the operating pressure is important for optimizing the values of GOR, FVF and API.
An exemplary operational objective function is J=Revenues−operational cost, where:
      Revenues    =                            sales          ⁢                                          ⁢          of          ⁢                                          ⁢          STO                +                  sales          ⁢                                          ⁢          of          ⁢                                          ⁢          gas                    =                                                  f              1                        ⁡                          (              API              )                                ×                                    P              wo                        FVF                          +                              f            2                    ⁡                      (                                          P                wo                            ⁢              GOR                        )                                ,and where
      P    wo    FVFis the production rate of the STO, ƒ1 (API) is the price of a barrel of oil as a function of oil API, and ƒ2(PwoGOR) is the sales price of the produced gas. The operational cost is also a function, ƒ3(Pwo), which represents the cost of a barrel as a function of oil well production.
It would be desirable to replace costly empirical testing, as described above, with an optimization method based on a user-defined overall operational cost function J=Revenues−operational cost, which could then be optimized by determining operating pressures that select the best values for FVF, GOR and API.
FIG. 4 illustrates a separator stage vessel 300 in greater detail than that shown in FIG. 2. Inlet flow is received via a pipe or conduit 302. The conditions Pin and Tin represent the pressure and temperature, respectively, of the incoming oil from the previous stage, or from the oil well if the stage is the first one. The collected oil is taken to the second stage through pipe 304, where the rate of flow is controlled by a control valve 306. The rate of oil flow is governed by a feedback control loop to maintain the oil level at a specified set point. The control loop contains a level sensor 310 and a controller 312. The controller takes the measured level value and compares it with the desired set point value 314, and calculates the adjustment position of the control valve 306 to change the oil flow to keep the level of oil in the vessel within the desired range. The stage pressure and temperature are denoted as Ps and Ts, respectively.
The stage temperature is measure by a temperature sensor 316. Ta is the ambient temperature, which directly affects the operation of the stage due to the heat loss to the ambient environment. The pressure of the stage Ps is controlled via a pressure control loop, where a pressure sensor 318 measures Ps, the stage pressure, and sends it to a controller 320. The controller compares the stage pressure with the desired set point pressure 322 of the stage and adjusts the gas flow via control valve 324. In the majority of GOSPs, the pressure set points are determined at the design stage and kept fixed during the plant operation. The ratio of the separated gas to the collected oil is the stage gas-to-oil ratio. The collected oil becomes the inlet to the next stage, and so on.
The released gas in every stage is a complex function of the flow rate, inlet temperature and pressure, along with the stage pressure and temperature. The stage temperature is similarly a complicated function of the above-mentioned parameters and fluctuates with the ambient temperature between day and night, and between summer and winter.
Thus, a method for optimizing and controlling pressure in gas-oil separation plants solving the aforementioned problems is desired.